Telecommunication systems comprising a number of optical transmission channels are well known in the art. The need to switch sometimes from one traffic carrying path to another arises e.g. when there is an occasional fault occurring in one of the traffic carrying channels, due to failing components, or when an expansion of the operating communication network is required. Therefore, an auxiliary channel is usually incorporated in such systems, allowing the diversion of traffic transmitted to another channel, which is sometimes referred to as the protection channel. Traditionally, monitoring the performance in these telecommunication systems was done by incorporating various alarm conditions. Such alarm conditions alerted a human operator when certain events e.g. a loss of signal or error rates that had exceeded pre-defined thresholds were detected. In response to such an alarm, the operator would manually switch to a redundant path in the network, allowing the communication to continue.
At a later stage, conventional fiber optic fibers have implemented 1:1 redundancy for the optical channels in a network, with a certain amount of automatic switching. In these systems, when a loss of signal (to be referred to hereinafter as “LOS”) or alarm indication signal (“AIS”) conditions were noted in a channel connecting two locations, a diversion of the transmission to the available redundant path was made. This diversion enables the transmission of data between these two locations to continue.
U.S. Pat. No. 4,646,286 discloses a system wherein a protection switch is effected by detecting a channel failure at the receiving end. Thereafter, a protection request is transmitted on the return channel towards the transmission end. This request is then used in a controller for the channel to activate a switch to the corresponding protection channel. Similarly, WO 97/24822 discloses the use of distinctive optical signals indicating detection of fault in order to allow a switch to a protection link.
However, since this solution requires doubling both the cabling and the input/output ports as compared with those required for carrying live traffic, such a solution is considered to be a rather expensive solution.
Another solution was described in U.S. Pat. No. 5,479,608 that discloses a telecommunication system having 1:N group protection. By this type of solution one redundant channel is allocated to protect a number of operative channels. According to this solution, in response to the detection of an error condition, a request is transmitted to the other side of the system to activate the protection channel.
Chung-Sheng Li in “Automatic Fault Detection, Isolation, and Recovery in Transport All-Optical Networks” J. of Lightwave Tech., IEEE, Vol. 15, 1784–1793, 1997 describes the use a separate supervisory channel using either independent wavelength or a subcarrier channel wherein a pilot tone provides information for fault management.